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          <h1 class="post-title" itemprop="name headline">javascript 单例模式</h1>
        

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        <p><strong>单例模式的定义是：保证一个类仅有一个实例，并提供一个访问它的全局访问点。</strong></p>
<p>单例模式能在合适的时候创建对象，并且创建唯一的一个。<br>代码接近于生活，很有意思。比如一个网站的登录，点击登录后弹出一个登录弹框，即使再次点击，也不会再出现一个相同的弹框。又或者一个音乐播放程序，如果用户打开了一个音乐，又想打开一个音乐，那么之前的播放界面就会自动关闭，切换到当前的播放界面。这些都是单例模式的应用场景。<br>要实现一个单例模式，一个经典的方式是创建一个类，类中又一个方法能创建该类的实例对象，还有一个标记，记录是否已经创了过了实例对象。如果对象已经存在，就返回第一次实例化对象的引用。</p>
<h1 id="单例模式的实现"><a href="#单例模式的实现" class="headerlink" title="单例模式的实现"></a>单例模式的实现</h1><h2 id="es5实现方式"><a href="#es5实现方式" class="headerlink" title="es5实现方式"></a>es5实现方式</h2><figure class="highlight actionscript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> Singleton = <span class="function"><span class="keyword">function</span><span class="params">(name)</span> </span>&#123;</span><br><span class="line">    <span class="keyword">this</span>.name = name;</span><br><span class="line">    <span class="comment">//一个标记，用来判断是否已将创建了该类的实例</span></span><br><span class="line">    <span class="keyword">this</span>.instance = <span class="literal">null</span>;</span><br><span class="line">&#125;</span><br><span class="line"><span class="comment">// 提供了一个静态方法，用户可以直接在类上调用</span></span><br><span class="line">Singleton.getInstance = <span class="function"><span class="keyword">function</span><span class="params">(name)</span> </span>&#123;</span><br><span class="line">    <span class="comment">// 没有实例化的时候创建一个该类的实例</span></span><br><span class="line">    <span class="keyword">if</span>(!<span class="keyword">this</span>.instance) &#123;</span><br><span class="line">        <span class="keyword">this</span>.instance = <span class="keyword">new</span> Singleton(name);</span><br><span class="line">    &#125;</span><br><span class="line">    <span class="comment">// 已经实例化了，返回第一次实例化对象的引用</span></span><br><span class="line">    <span class="keyword">return</span> <span class="keyword">this</span>.instance;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>用户可以通过一个广为人知的接口，对该实例进行访问。<br>我们尝试对该对象进行两次实例化，观察两次实例化结果是否指向同一个对象。</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> a = Singleton.getInstance(<span class="string">'sven1'</span>);</span><br><span class="line"><span class="keyword">var</span> b = Singleton.getInstance(<span class="string">'sven2'</span>);</span><br><span class="line"><span class="comment">// 指向的是唯一实例化的对象</span></span><br><span class="line"><span class="built_in">console</span>.log(a === b);</span><br></pre></td></tr></table></figure>
<p>返回结果是：true。说明a、b之间是引用关系。</p>
<h2 id="es6实现方式"><a href="#es6实现方式" class="headerlink" title="es6实现方式"></a>es6实现方式</h2><p>创建Singleton类。class关键字和静态函数都是es6新增的。</p>
<figure class="highlight kotlin"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Singleton</span> </span>&#123;</span><br><span class="line">    <span class="keyword">constructor</span>(name) &#123;</span><br><span class="line">        <span class="keyword">this</span>.name = name;</span><br><span class="line">        <span class="keyword">this</span>.instance = <span class="literal">null</span>;</span><br><span class="line">    &#125;</span><br><span class="line">    <span class="comment">// 构造一个广为人知的接口，供用户对该类进行实例化</span></span><br><span class="line">    static getInstance(name) &#123;</span><br><span class="line">        <span class="keyword">if</span>(!<span class="keyword">this</span>.instance) &#123;</span><br><span class="line">            <span class="keyword">this</span>.instance = new Singleton(name);</span><br><span class="line">        &#125;</span><br><span class="line">        <span class="keyword">return</span> <span class="keyword">this</span>.instance;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<h2 id="typescript实现方式"><a href="#typescript实现方式" class="headerlink" title="typescript实现方式"></a>typescript实现方式</h2><figure class="highlight cs"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">class</span> <span class="title">Singleton</span> &#123;</span><br><span class="line">     </span><br><span class="line">  <span class="function"><span class="keyword">private</span> <span class="title">constructor</span> (<span class="params">name: <span class="keyword">string</span></span>)</span> &#123;</span><br><span class="line">    <span class="keyword">this</span>.name = name</span><br><span class="line">   </span><br><span class="line">  &#125;</span><br><span class="line"></span><br><span class="line">   <span class="keyword">private</span> <span class="keyword">static</span> instance: Singleton</span><br><span class="line">   <span class="keyword">public</span> name: <span class="keyword">string</span>  </span><br><span class="line"></span><br><span class="line">   </span><br><span class="line">  <span class="function"><span class="keyword">static</span> <span class="title">getInstance</span> (<span class="params">name: <span class="keyword">string</span></span>): Singleton</span> &#123;</span><br><span class="line">    <span class="keyword">if</span> (!<span class="keyword">this</span>.instance) &#123;</span><br><span class="line">      <span class="keyword">this</span>.instance = <span class="keyword">new</span> Singleton(name)</span><br><span class="line">    &#125;</span><br><span class="line">    <span class="keyword">return</span> <span class="keyword">this</span>.instance</span><br><span class="line">  &#125;</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"></span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<h1 id="单例模式应用实例"><a href="#单例模式应用实例" class="headerlink" title="单例模式应用实例"></a>单例模式应用实例</h1><p>我们用一个生活中常见的一个场景来说明单例模式的应用。<br>任意一个网站，点击登录按钮，只会弹出有且仅有一个登录框，即使后面再点击登录按钮，也不会再弹出多一个弹框。这就是单例模式的典型应用。接下来我们实现它。为了注重单例模式的展示，我们把登录框简化吧😜</p>
<p>在页面上设置一个按钮</p>
<p><code>&lt;button id=&quot;loginBtn&quot;&gt;登录&lt;/button&gt;</code></p>
<p>为这个按钮添加点击事件</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">const</span> btn = <span class="built_in">document</span>.getElementById(<span class="string">'loginBtn'</span>);</span><br><span class="line">btn.addEventListener(<span class="string">'click'</span>, <span class="function"><span class="keyword">function</span>(<span class="params"></span>) </span>&#123;</span><br><span class="line">    loginLayer.getInstance();</span><br><span class="line">&#125;, <span class="literal">false</span>);</span><br></pre></td></tr></table></figure>
<p>我们先给一个初始化方法init，在点击按钮之后，在页面上添加一个框框（权当登录框了）</p>
<figure class="highlight cal"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line">init() &#123;</span><br><span class="line">    <span class="keyword">var</span> <span class="keyword">div</span> = document.createElement(<span class="string">'div'</span>);</span><br><span class="line">    <span class="keyword">div</span>.classList.add(<span class="string">'login-layer'</span>);</span><br><span class="line">    <span class="keyword">div</span>.innerHTML = <span class="string">"我是登录浮窗"</span>;</span><br><span class="line">    document.body.appendChild(<span class="keyword">div</span>);</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>那么接下来要用单例模式实现，点击按钮出现有且仅有一个浮窗，方法就是在构造函数中创建一个标记，第一次点击时创建一个浮窗，用改变标记的状态，再次点击按钮时，根据标记的状态判断是否要再创建一个浮窗。<br>上源码：</p>
<figure class="highlight kotlin"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">loginLayer</span> </span>&#123;</span><br><span class="line">    <span class="keyword">constructor</span>() &#123;</span><br><span class="line">        <span class="comment">// 判断页面是否已有浮窗的标记</span></span><br><span class="line">        <span class="keyword">this</span>.instance = <span class="literal">null</span>;</span><br><span class="line">        <span class="keyword">this</span>.<span class="keyword">init</span>();</span><br><span class="line">    &#125;</span><br><span class="line">    <span class="keyword">init</span>() &#123;</span><br><span class="line">        <span class="keyword">var</span> div = document.createElement(<span class="string">'div'</span>);</span><br><span class="line">        div.classList.add(<span class="string">'login-layer'</span>);</span><br><span class="line">        div.innerHTML = <span class="string">"我是登录浮窗"</span>;</span><br><span class="line">        document.body.appendChild(div);</span><br><span class="line">    &#125;</span><br><span class="line">    <span class="comment">// 静态方法作为广为人知的接口</span></span><br><span class="line">    static getInstance() &#123;</span><br><span class="line">        <span class="comment">// 根据标记的状态判断是否要再添加浮窗，如果标记不为空就创建一个浮窗</span></span><br><span class="line">        <span class="keyword">if</span>(!<span class="keyword">this</span>.instance) &#123;</span><br><span class="line">            <span class="keyword">this</span>.instance = new loginLayer();</span><br><span class="line">        &#125;</span><br><span class="line">        <span class="keyword">return</span> <span class="keyword">this</span>.instance;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>当然不要忘记为浮窗添加一个样式，让浮窗显示啦</p>
<figure class="highlight css"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="selector-class">.login-layer</span> &#123;</span><br><span class="line">    <span class="attribute">width</span>: <span class="number">200px</span>;</span><br><span class="line">    <span class="attribute">height</span>: <span class="number">200px</span>;</span><br><span class="line">    <span class="attribute">background-color</span>: <span class="built_in">rgba</span>(0, 0, 0, .6);</span><br><span class="line">    <span class="attribute">text-align</span>: center;</span><br><span class="line">    <span class="attribute">line-height</span>: <span class="number">200px</span>;</span><br><span class="line">    <span class="attribute">display</span>: inline-block;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>最后奉上该实例的源码：</p>
<figure class="highlight kotlin"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br></pre></td><td class="code"><pre><span class="line">   loginLayer.getInstance();</span><br><span class="line">        &#125;, <span class="literal">false</span>);</span><br><span class="line">        </span><br><span class="line">        <span class="class"><span class="keyword">class</span> <span class="title">loginLayer</span> </span>&#123;</span><br><span class="line">            <span class="keyword">constructor</span>() &#123;</span><br><span class="line">                <span class="keyword">this</span>.instance = <span class="literal">null</span>;</span><br><span class="line">                <span class="keyword">this</span>.<span class="keyword">init</span>();</span><br><span class="line">            &#125;</span><br><span class="line">            <span class="keyword">init</span>() &#123;</span><br><span class="line">                <span class="keyword">var</span> div = document.createElement(<span class="string">'div'</span>);</span><br><span class="line">                div.classList.add(<span class="string">'login-layer'</span>);</span><br><span class="line">                div.innerHTML = <span class="string">"我是登录浮窗"</span>;</span><br><span class="line">                document.body.appendChild(div);</span><br><span class="line">            &#125;</span><br><span class="line">            static getInstance() &#123;</span><br><span class="line">                <span class="keyword">if</span>(!<span class="keyword">this</span>.instance) &#123;</span><br><span class="line">                    <span class="keyword">this</span>.instance = new loginLayer();</span><br><span class="line">                &#125;</span><br><span class="line">                <span class="keyword">return</span> <span class="keyword">this</span>.instance;</span><br><span class="line">            &#125;</span><br><span class="line">        &#125;</span><br><span class="line">    &lt;/script&gt;</span><br><span class="line">&lt;/body&gt;</span><br><span class="line">&lt;/html&gt;</span><br></pre></td></tr></table></figure>

      
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